1978 911 SC sputtering / bucking in 3rd and 4th gear
01-20-2019, 08:38 PM
#1
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1978 911 SC sputtering / bucking in 3rd and 4th gear
I have a 78 911 SC, all stock with a recent top end rebuild,ˇ when driving at freeway speeds (50+ MPH), car will sputter / buck between 2800 to 3300 RPM in 3rd and 4th gear, the shifter will shake and rattle, this happens when I'm light on the throttle, if I apply a little more gas, car doesn't sputter, car doesn't sputter out of this RPM range or when going faster
My mechanic thought this was due to CIS mixture and has adjusted it several times w/o success, would really appreciate any insight / guidance on how to resolve this issue.
Here's a video of the shifter bucking
https://photos.app.goo.gl/9nobEwGz6xMbhDFW8
Thanks!
My mechanic thought this was due to CIS mixture and has adjusted it several times w/o success, would really appreciate any insight / guidance on how to resolve this issue.
Here's a video of the shifter bucking
https://photos.app.goo.gl/9nobEwGz6xMbhDFW8
Thanks!
01-21-2019, 05:28 PM
#4
I'm going to go way out on a limb and say it's some harmonic developing in that rpm range where there's just enough throttle to load up the transmission but all the mounts are so loose that it's vibrating the whole driveline between load and unload. I agree with Starbuckslova, motor/trans mounts and then connections between shifter and trans. That it's happening with 3rd and 4th gear, which are in the neutral position with the trans, seems like an indicator.
01-21-2019, 06:07 PM
#5
Team Owner
hot tip , when doing serious troubleshooting , turn off the music so people can hear the engine , couldn't tell anything because of the music,
01-21-2019, 08:22 PM
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Track Day
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Thank you! I just talked to him today and will let him know to check the mounts, he was speculating it might be a short in the CD unit.
JFYI, he adjusted the mixture and the frequency of the issue def. went down (from 75% to 20% of the time) so he thought there was a correlation.
But if it was the mounts, wouldn't this occur at all RPM range and not just between 2.8 and 3.3?
JFYI, he adjusted the mixture and the frequency of the issue def. went down (from 75% to 20% of the time) so he thought there was a correlation.
But if it was the mounts, wouldn't this occur at all RPM range and not just between 2.8 and 3.3?
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01-24-2019, 06:58 PM
#8
Rennlist Member
VIdeo link doesn't work. Error message.
02-10-2019, 08:28 PM
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Thank you everyone for all the advice, the root cause does appear to be the motor / trans mounts which have now been replaced, I wanted to drive the car for a bit to verify as the problem was intermittent before, I've now driven on the free way ~50 miles in 3rd and 4th in the same RPM range where the shiftier used to buck and it no longer does.
